The present invention relates to an accessory for a nutrunner for making the same particularly efficient for use in an automated assembly/disassembly system. More particularly, the present invention is a bolt retaining device in the form of an electromagnetic collar for the socket for an automated nutrunner.
Nutrunners are devices that are generally known in the art both as hand tools and as motor-driven tools for applying and securing nuts on bolts or removing nuts from bolts or for driving and securing bolts or removing bolts to assemble and/or disassemble workpieces.
During assembly and disassembly processes, it is sometimes necessary to remove bolts from the workpiece being assembled or disassembled. During an automated or a semi-automated process, machines with motor driven sockets are used to loosen or tighten bolts on the workpiece being assembled or disassembled as that workpiece moves down the assembly line. The nutrunner generally includes a motor, a socket with an appropriate size opening for the bolt to be loosened or driven with the motor being connected to the socket via a flexible extensible shaft Often plural nutrunners are mounted on the same stand with the plural sockets being carried by a frame. The frame is independently movable toward and away from the workpiece rolling along the assembly line. The assembly line can be either a continuous flow process in which the plural nutrunners on the frame are carried along at the same speed as the workpiece for as long as it takes to perform the desired operation or the assembly line can be a station by station line in which the workpiece moves into a particular station and remains there until the operations to be performed there are completed. In any event, the socket of the nutrunner through other operating structure is moved to advance toward the workpiece having the bolt to be tightened or removed and is movable away from the workpiece.
In certain assembly line processes, workpieces such as internal combustion engines are assembled at a separate location and brought to the assembly line to be assembled with the additional components and then assembled into the vehicle. For protective purposes, openings on the internal combustion engine where later components are to be assembled, are covered by a cap during shipment or movement from the engine assembly plant to the vehicle assembly line. Such a cap could be the cap covering the opening at the end of the engine adjacent the main bearing where the crankshaft is designed to be connected to the clutch or transmission. Such caps are typically held in place by bolts that are engaged in the threaded holes that the final assembly bolts will engage. Such bolts must be loosened and taken out and the cap removed prior to the engine being assembled with the remaining components. Generally, plural bolts are provided for holding on a specific cap. Each of these bolts must be loosened and removed in order to remove the cap prior to the subsequent assembly of the engine with the other components. Originally the bolts were loosened and removed by pneumatically-driven hand-held nutrunners by individual workers.
In a more modem plant, as the engine moves down the assembly line, the motor driven socket machine advances the motor driven sockets onto the bolts to be loosened. The engine is appropriately positioned and the sockets are appropriately mounted on the frame to mate with a single movement of the sockets toward the bolts. The individual motors are turned on for a specific amount of time required to loosen the bolt. The frame of the motor driven socket machine then retracts from the engaged position and disengages the sockets from the loosened bolts. The engine then moves further down the assembly line where the loosened bolt(s) are removed by a worker.
During the loosening of the bolt, the head of the bolt moves within or along inside the bore of the socket or the socket moves slowly backwardly against the biasing of a spring as the bolt unthreads out of the hole. At present, there is no secure structure for holding the bolt within the socket upon the retreat of the plural socket carrying frame should the bolt be loosened far enough to come out of the hole. This creates a problem of bolts dropping to the floor of the assembly line if they have been unloosened too far.
Various socket type hand tools are known in which a permanent magnet is provided for holding the nut or bolt head therein to facilitate putting it into an operable position by manipulating the wrench or handle upon which the socket is fitted. Examples are shown in U.S. Pat. No. 4,663,998 to Parsons et al, U.S. Pat. No. 5,603,248 to Eggert et al or U.S. Pat. No. 5,916,340 to Forsyth. In each of these, a permanent magnet is held at the base of the bore of a socket. The magnet can be held in place by a C-clip in a peripheral wall groove formed in the inner circumference of the socket, or by a circular retainer interference fit in the base of the bore of the socket above the magnet or by being imbedded or connected to a socket wrench connector tip insertable in a driver bore of a manual socket.
One of the difficulties in using a nut driver in an automated or semi-automated process for driving a bolt or nut is to be able to sense or measure the amount of torque being applied to the bolt or nut. It is often desired to have a specific preload on a bolt or nut. Bolts having an improper preload can lead to a catastrophic failure of a critical component. One known method is to monitor the applied torque on the socket such as in the system shown in U.S. Pat. No. 4,987,806 to Lehnert. The torque is monitored by a strain gauge affixed to the drive shaft of the motor and arranged to provide a signal proportional to the applied torque. Another structure is taught in U.S. Pat. No. 5,499,540 to Whaley et al in which a device for measuring the load on the bolt includes an electromagnetic acoustic transducer comprising a magnet and a coil located in the interior space of the socket near the bolt head. The signal is taken out of the coil by a slip ring assembly engaged around the socket drive. The magnet provided is a permanent magnet.
Providing a permanent magnet in a socket of a conventional nutrunner would require special design and expensive retooling of standard sockets. Further, providing a permanent magnet in a socket of a conventional nutrunner may assist in assembling nuts onto a stud or assembling bolts into a unit moving along an assembly line by preholding the nuts or bolts and thereafter advancing the nutrunner into position. However, such is not the case with a nutrunner used for disassembly.
In designing an automated or a semi-automated disassembly process, various factors need to be considered. The particular bolts encountered may have variations in the bolt head such that the provision of a permanent magnet may not provide a magnet strong enough to hold the bolt when the bolt is completely removed from the hole. Further, difficulties may be encountered due to contamination on the bolt head becoming clogged or stuck in the socket and weakening the magnetic field by distancing the bolt head from the permanent magnet. Any attempt to increase the strength or size of the permanent magnet would increase the difficulty of releasing or removing the bolt from the socket following the disassembly of the bolt from the workpiece on the assembly line. A further mechanism, such as a pin push release mechanism, would be required. Further, variations in bolt length due to changes in the assembly line would require changes in the permanent magnet so that sufficient magnetic field strength is provided to hold the bolt in the socket. Thus, not only would specially designed and provided sockets be required, a plurality of field strengths for the same size socket would be required and multiple socket shifts would be needed as product lines were changed.